The present invention relates to a method for detecting contamination on a moving object moving in a longitudinal direction past a plurality of detectors.
It is customary to perform inspections for the transport of radioactive sources at the entrances and exits of nuclear facilities, but also at border crossings, airports, or in general, at entrances and exits of buildings or regions. The inspections performed serve, on the one hand, for protecting people and on the other hand, can also uncover the illegal transport of radioactive material. These two aspects for detecting contamination are combined in the following when speaking of objects to be detected or inspected. Typically the objects to be inspected, which can be people, freight and/or vehicles, are led through a so-called portal monitor, in which detectors for gamma radiation and/or gamma and neutron radiation are provided transverse to the direction of movement.
In the case of a steady flow of objects to be inspected, the portal monitor used can become a chokepoint at which a bottleneck forms. In the case of inspecting people at entrances and exits, wait times arise. Also with container inspections, for example, at ports, the wait time can result in a slow-down in the processing of containers. False alarms, in which the portal monitor wrongly indicates contamination, are a possible cause of such wait times.
An object of the invention is to provide a method and a measurement apparatus for detecting contamination on a moving object that with the most simple means possible avoids false alarms and allows an exact measurement.
With the method according to the invention, the contamination is detected on an object moving in a longitudinal direction past a plurality of detectors. According to the invention, a count rate is recorded repeatedly by each detector during the movement of the object past the detectors disposed consecutively in the longitudinal direction. The recorded count rates are subjected to a validity check before an evaluation to determine whether contamination is present. The evaluation of the recorded count rates can occur in a known manner. For the validity check, count rates recorded by the detectors are compared to a class of pre-determined reference patterns for the count rates. A temporal progression of the count rate originates at each of the detectors as a result of the repeated recording of the count rate at the detectors. This temporal progression of the count rates is considered as a pattern and can be compared to predetermined reference patterns. Algorithms for pattern detection that compare congruence in the patterns, but are not dependent on concrete values of count rates, can be used here. Comparing to the reference patterns can ensure that only plausible count rates are supplied for evaluation with regard to a radiation exposure. If during the validity testing it is recognized that the recorded sample does not belong to a class of predetermined reference patterns, a signal can be generated that an error is present in the recorded count rates. This makes it necessary to repeat the measurement procedure for the object. If the validity testing determines that the recorded count rates are plausible these can be evaluated in order to attained a reliable measurement result. False alarms are avoided using the validity testing because only plausible measurement data are evaluated. Apart from that, the results of the evaluation of the count rates are also improved because only plausible count rates are evaluated. It can also be provided that specific count rates which were determined as plausible are supplied to a particular evaluation.
In a preferred further development of the method, the class of reference patterns comprises the temporal sequence in which maxima in the count rates have occurred at the detectors. The reference pattern can contain, for example, as a temporal sequence of the detectors 1, 2, 3, 4 . . . , wherein the implication of the reference pattern is that a maximum has occurred temporally consecutively in the count rates at the detectors 1, 2, 3, 4 . . . . With this further development of the method, for the validity testing the temporal sequence of detectors, in which maxima have occurred in the count rates, is determined from the recorded count rates. An error is then detected in the validity testing if the temporal sequence of the detectors determined from the recorded count rates is not contained in the class of the reference patterns. By comparing the class of the reference patterns, cases can therefore be excluded in which, for example, a detector located further to the rear already indicates a maximum that should have occurred only later, when the object has reached this detector. The validity testing then indicates that implausible count rates were recorded, and generates a corresponding warning signal.
In a further preferred embodiment, the temporal progression of the recorded count rates at the individual detectors is compared to the temporal progression of reference patterns using pattern detection. With the use at portal monitors, in which an object with or without sources of radiation is moved past the detectors in the longitudinal direction, a characteristic temporal progression of count rates arises in the case of the presence of a radioactive radiation. If the recorded temporal progression of count rates at one of the detectors deviates from the progressions of the reference patterns, it can be recognized that non-plausible count rates are present.
In a preferred embodiment, the class of reference patterns also comprises count rates, in which in at least one of the detectors there is a reduction of the count rate below an average value of the background count rate. This phenomenon occurs when a massive object moves past the detectors. In this case, a shielding of the background radiation occurs so that the background count rate is initially reduced. Preferably the class of the reference pattern comprises also count rates in which at one detector there is an increase in the count rate after a reduction below the average value of the background count rate. This is the case, for example, when a massive object is located in front of the detector because then the background radiation is initially shielded, and subsequently the increased net count rate due to the radiation source is recorded.
In a preferred further development of the method according to the invention, when the count rates of the individual detectors indicate a lowering beneath the value of an average background count rate, it is adjusted to the value of the increase of the count rate, for evaluating the count rate. With the help of the validity testing it can be ensured here that count rates with this characteristic progression are present, so that this is taken into consideration during the evaluation.